PALEOGENE DEXTRAL TRANSTENSION AND DUCTILE CRUSTAL BOUDINAGE: LINKING LISTRIC NORMAL FAULTING IN THE CORDILLERAN FORELAND THRUST-AND-FOLD BELT TO TECTONIC EXHUMATION OF MID-CRUSTAL METAMORPHIC INFRASTRUCTURE IN THE HINTERLAND

The depth to the Moho decreases abruptly westward from >45 km to <37.5 km near the southern Rocky Mountain trench, but is relatively constant at ~35 km westward from there to the Coast Mountains. Eocene extensional collapse of the Cordilleran orogen involved crustal scale boudinage. It occurred during an episode of pervasive dextral lithospheric transtension that was localized within the region of en echelon overlap linking the Tintina-Northern Rocky Mountain trench fault zone, first to the Yalakom-Ross Lake fault zone, and then to the Fraser-River Straight Creek fault zone. En echelon tectonically exhumed crustal boudins that comprise ~ 35 km of mid- to lower crustal rock with only vestiges of overlying supracrustal rock stand in stark contrast with adjacent crustal neck zones in which the mid- and lower crust is only ~15 km thick and the overlying supracrustal rocks show no evidence of comparable horizontal extension. This pattern of reciprocal upper and lower crustal extension involved detachments that were controlled by sub-horizontal rheological layering produced during the preceding episode of Late Cretaceous-Paleocene dextral transpression, which also was linked northward to the Tintina-Northern Rocky Mountain trench fault zone. Within the metamorphic core of the orogen, detachment occurred across a low-ductility zone of mid-crustal migmatization that developed in the “root zone” of the Cordilleran foreland thrust and fold belt during the progressive Cretaceous-Paleocene crustal thickening of over-riding allochthonous overthrust rocks and the progressive under-riding of the autochthonous Paleoproterozoic and Archean crystalline basement that extends under the Cordillera from the Canadian Shield. During boudinage of the lower crust, overlying supracrustal rocks that were detached and stretched above the boudins moved down toward the neck zones produced by reciprocal stretching of the lower crust. Similarly, thin-skin normal faulting in the Rocky Mountains that involved reactivation of pre-existing thrust faults, including the regional basal detachment of the foreland thrust and fold belt, also was transferred to reciprocal ductile stretching of lower crust in a neck zone; but stratigraphic evidence indicates that at least some of the extension of the southern Rocky Mountains occurred later.